A polymer electrolyte fuel cell in which a hydrogen-containing fuel gas and oxygen-containing oxidizing gas are supplied to an anode and cathode, respectively, and an electromotive force is generated by bipolar electrochemical reaction is generally constituted of sequentially laminating a bipolar plate, a gas diffusion electrode medium, a catalyst layer, an electrolyte membrane, a catalyst layer, a gas diffusion electrode medium and a bipolar plate. The gas diffusion electrode medium is required to have high gas diffusibility in an in-plane direction and high gas diffusibility in a direction perpendicular to a plane to allow a gas supplied from the bipolar plate to be diffused into the catalyst layer and a high drainage property to drain liquid water generated by an electrochemical reaction to the bipolar plate as well as high electrical conductivity to extract a generated electric current, and electrode base materials composed of carbon fibers and the like are widely used.
However, the following problems are known: (1) when such a polymer electrolyte fuel cell is operated at a relatively low temperature of below 70° C. in a high current density region, as a result of blockage of the electrode base material by liquid water generated in a large amount and shortage in the fuel gas supply, the cell performance is impaired (this problem is hereinafter referred to as “flooding”); and (2) when such a polymer electrolyte fuel cell is operated at a relatively high temperature of 80° C. or higher, as a result of drying of the electrolyte membrane due to water vapor diffusion and a reduction in the proton conductivity, the cell performance is impaired (this problem is hereinafter referred to as “dry-out”). Various efforts have been made to solve these problems of (1) and (2).
Japanese Patent Laid-open Publication No. 2000-123842 proposes a gas diffusion electrode medium in which a microporous layer composed of a carbon black and a water-repelling resin is formed on the catalyst layer side of an electrode base material with a high areal weight of 167 g/m2. According to a fuel cell comprising this gas diffusion electrode medium, since the microporous layer has a fine pore structure having water repellency, drainage of liquid water to the cathode side is inhibited so that flooding tends to be inhibited. In addition, since generated water is forced back to the electrolyte membrane side (hereinafter, this phenomenon is referred to as “back-diffusion”), the electrolyte membrane is wetted and the problem of dry-out thus tends to be inhibited. However, there is a problem that inhibition of flooding and dry-out is still insufficient.
Japanese Patent Laid-open Publication No. 2004-311431 proposes a gas diffusion electrode medium in which a microporous layer composed of a carbon black and a water-repelling resin is formed on the catalyst layer side of an electrode base material with from a relatively low areal weight to a relatively high areal weight of 44 to 92 g/m2. It had been expected that in accordance with fuel cells using these gas diffusion electrode medium, flooding would be inhibited due to the improvement of the gas diffusibility of the electrode base material and the drainage properties. However, inhibition of the flooding is still insufficient and there is a problem that dry-out cannot be inhibited.
Japanese Patent Laid-open Publication No. 2006-120506 proposes a gas diffusion electrode medium in which a microporous layer composed of a carbon black, a linear carbon and a water-repelling resin is formed on the catalyst layer side of an electrode base material with a relatively high areal weight of 84 g/m2. It had been expected that in accordance with a fuel cell using this gas diffusion electrode medium, flooding would be inhibited due to the improvement of the gas diffusibility of the microporous layer and the drainage properties. However, inhibition of the flooding is still insufficient and there is a problem that dry-out cannot be inhibited.
As described above, various efforts have been made; however, a satisfactory gas diffusion electrode medium which has an excellent anti-flooding characteristic and excellent anti-dry-out characteristic is yet to be discovered.
It could therefore be helpful to provide a gas diffusion electrode medium excellent in its anti-flooding and anti-dry-out characteristics and capable of exerting high cell performance across a wide temperature range from low to high temperatures and has excellent mechanical properties, electrical conductivity and thermal conductivity.